Field of the Invention
The present invention relates to a sputtering target for use in producing a Cu—In—Ga—Se compound film (hereinafter may be abbreviated to “CIGS film”) for forming the light-absorbing layer of a GIGS thin-film solar cell, and a method for producing the same.
Description of the Related Art
In recent years, thin-film solar cells, including chalcopyrite-based compound semiconductors, have come into practical use. The thin-film solar cells including the compound semiconductors have a basic structure in which an Mo electrode layer (positive electrode) is formed on a soda-lime glass substrate, a light-absorbing layer made of a GIGS film is formed on the Mo electrode layer, a buffer layer of ZnS, CdS, or the like is formed on the light-absorbing layer, and a transparent electrode layer (negative electrode) is formed on the buffer layer.
As a method for forming light-absorbing layers as described above, a multi-source deposition method, for example, is known. This method can produce light-absorbing layers having a high energy conversion efficiency. However, its vapor deposition from a point source tends to reduce the uniformity of film thickness distribution when the deposition is performed on a large-area substrate. Thus, a sputtering method has been proposed for forming light absorbing layers.
In the sputtering method (what is called “selenization method”) for forming light-absorbing layers as described above, first an In film is deposited by sputtering using an In target, and then a Cu—Ga binary alloy film is deposited or this In film by sputtering using a Cu—Ga binary alloy target; and the resultant laminated precursor film consisting of the In film and the Cu—Ga binary alloy film is subject to a heat treatment in a Se atmosphere to produce a CIGS film.
Another method based on the above technique is also disclosed, in which said laminated precursor film, consisting of the Cu—Ga alloy film and the In film, is formed by depositing a Cu—Ga alloy layer with a high Ga content, a Cu—Ga alloy layer with a low Ga content, and an In layer in this order from a metal back electrode layer side by a sputtering method, and this film is subject to a heat treatment in a selenium and/or sulfur atmosphere so as to attain a Ga concentration gradient, which gradually (stepwise) changes from the interface layer (buffer layer) toward the metal back electrode layer, in the thin film light-absorbing layer. With this technique, the thin-film solar cell with a high open circuit voltage can be achieved, and peeling-off of the thin film light-absorbing layer from the other layer can be prevented. In this case, the Ga content in the CuGa target is 1 to 40 at % (see Patent document 1).
in order to form such a Cu—Ga alloy layer, Patent document 2 discloses use of a Cu—Ga alloy sintered compact sputtering target as a CuGa target that is produced by sintering a Cu—Ga mixed powder, which is prepared by a water atomization apparatus, by hot pressing, for example. This Cu—Ga alloy sintered compact sputtering target has a single composition, except the main peak (γ phase (Cu9Ga4 phase)), exhibits a peak intensity of 5% or less relative to the main peak in X-ray diffraction of the Cu—Ga alloy, and has an average crystal grain size of 5 to 30 μm. This target also has an oxygen content of 350 to 00 ppm.